Can't update Mongoose subdocument property - node.js

Question

Using the following line of code I can update a given Transaction (document), but when I try to update properties of its subdocument, the given value is not persisted.
Transaction.findById('55cf89abe148323e5368dcd5').populate('cryptocurrencies')
.then(function(transaction){
transaction.status = 'completed'; // this updates the transaction status correctly
transaction['cryptocurrencies'][0].status = 'ordered'; // this update is not persisted
return transaction.save()
.then(function(transaction){
console.log(transaction['cryptocurrencies'][0].status); // this shows the status as updated, but it's not persisted
})
})
I've also used the line transaction['cryptocurrencies'][0].markModified('status'); after I update the property to no avail. Can't find anything in the docs: What am I missing?
Update: I've tested this further and found that I have to use the .save() method on both the document and its subdocument. Is there any I can run a method that will save the document with its subdocument properties changed, or do I have to run two operations to save one document each time?
Update:
Here is my model code:
'use strict';
var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var TransactionSchema = new Schema({
userId: { type: String, required: true },
status: { type: String, enum: ['unpaid', 'failed', 'paid', 'ordered', 'received', 'withdrawn', 'completed'] },
invoice: String,
saltStatus: String,
saltTransactionId : Number,
saltBank: String,
saltConfirmation: String,
saltAmount : Number,
saltDate : String,
saltResponseCode: String,
cryptocurrencies: [{ type: mongoose.Schema.Types.ObjectId, ref: 'CryptoCurrency' }]
});
var CryptoCurrencySchema = new Schema({
currencyName: String,
price: Number,
amount: Number,
total : Number,
walletAddress: String,
dollarsPaid : Number,
exchangeTransactionId: Number,
coinTransactionId : String,
status: { type: String, enum: ['ordered', 'received', 'withdrawn'] }
});
module.exports.Transaction = mongoose.model('Transaction', TransactionSchema);
module.exports.CryptoCurrency = mongoose.model('CryptoCurrency', CryptoCurrencySchema);

Answer 1

From your code example it seems that you are saving the sub-documents as references which means that once you update the sub-document, you only need to call .save() for it and not the parent document as well.
If you are saving your documents as sub-schemas, once updating the sub-document you can call .save() only for the parent document and it will persist the child document as well.
From the docs:
Sub-documents enjoy all the same features as normal documents. The
only difference is that they are not saved individually, they are
saved whenever their top-level parent document is saved.
http://mongoosejs.com/docs/subdocs.html